U.S. patent number 7,100,145 [Application Number 10/938,510] was granted by the patent office on 2006-08-29 for method of identifying an extreme interaction pitch region, methods of designing mask patterns and manufacturing masks, device manufacturing methods and computer programs.
This patent grant is currently assigned to ASML Masktools B.V.. Invention is credited to Jang Fung Chen, Duan-Fu Stephen Hsu, Xuelong Shi.
United States Patent |
7,100,145 |
Shi , et al. |
August 29, 2006 |
Method of identifying an extreme interaction pitch region, methods
of designing mask patterns and manufacturing masks, device
manufacturing methods and computer programs
Abstract
Optical proximity effects (OPEs) are a well-known phenomenon in
photolithography. OPEs result from the structural interaction
between the main feature and neighboring features. It has been
determined by the present inventors that such structural
interactions not only affect the critical dimension of the main
feature at the image plane, but also the process latitude of the
main feature. Moreover, it has been determined that the variation
of the critical dimension as well as the process latitude of the
main feature is a direct consequence of light field interference
between the main feature and the neighboring features. Depending on
the phase of the field produced by the neighboring features, the
main feature critical dimension and process latitude can be
improved by constructive light field interference, or degraded by
destructive light field interference. The phase of the field
produced by the neighboring features is dependent on the pitch as
well as the illumination angle. For a given illumination, the
forbidden pitch region is the location where the field produced by
the neighboring features interferes with the field of the main
feature destructively. The present invention provides a method for
determining and eliminating the forbidden pitch region for any
feature size and illumination condition. Moreover, it provides a
method for performing illumination design in order to suppress the
forbidden pitch phenomena, and for optimal placement of scattering
bar assist features.
Inventors: |
Shi; Xuelong (Santa Clara,
CA), Chen; Jang Fung (Cupertino, CA), Hsu; Duan-Fu
Stephen (Fremont, CA) |
Assignee: |
ASML Masktools B.V. (Veldhoven,
NL)
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Family
ID: |
34119633 |
Appl.
No.: |
10/938,510 |
Filed: |
September 13, 2004 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20050034096 A1 |
Feb 10, 2005 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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10083683 |
Feb 27, 2002 |
6792591 |
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09840305 |
Apr 24, 2001 |
6519760 |
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Current U.S.
Class: |
716/53 |
Current CPC
Class: |
G03F
7/70125 (20130101); G03F 7/70433 (20130101); G03F
7/70441 (20130101); G03F 7/705 (20130101) |
Current International
Class: |
G06F
17/50 (20060101) |
Field of
Search: |
;716/19-21 ;430/5 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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6005487 |
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Jan 1994 |
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JP |
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6120114 |
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Apr 1994 |
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JP |
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6163350 |
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Jun 1994 |
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JP |
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6181168 |
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Aug 1994 |
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JP |
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6301192 |
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Oct 1994 |
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JP |
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7134390 |
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May 1995 |
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JP |
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WO 98/40791 |
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Sep 1998 |
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WO |
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Other References
Kim H-J Et Al: "Fabrication of Dense Contact Partterns Using
Halftone Phase-Shifting Mask With Off-Axis Illumination"
Proceedings of the SPIE. SPIE, Bellingham, VA., US, vol. 2793, Jun.
18, 1996, pp. 106-114, XPOO9015384 ISSN: 0277-786X. cited by other
.
"Electromagnetic diffraction in optical systems I. An integral
representation of the image field", E. Wolf, (1959). cited by other
.
"Forbidden Pitches for 130nm lithography and below", Robert Socha
et al., Optical Microlithgraphy XIII, Proceedings of SPIE, vol.
4000 (2000). cited by other .
"Theory of high-NA imaging in homogeneous thin films", Donis G.
Flagello et al., vol. 13, No. 1/ (Jan. 1996), J. Opt. Soc. Am. A.
cited by other .
"Extension of the Hopkins theory of partially coherent imaging to
include thin-film interference effects", Michael S. Yeung et al.,
SPIE vol. 1927, Optical/Laser Microlithography V1 (1993). cited by
other .
"An OPC Technology Roadmap to 0.14.mu.m Design Rules", J. Fung Chen
et al., SPIE vol. 3236. cited by other .
"Fabrication of Dense Contact Patterns Using Halftone
Phase-Shifting Mask With Off-Axis Illumination", Hyoung-Joon Kim et
al., SPIE vol. 2793. cited by other.
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Primary Examiner: Dinh; Paul
Attorney, Agent or Firm: McDermott Will & Emery LLP
Parent Case Text
This application is a continuation application of application Ser.
No. 10/083,683 filed Feb. 27, 2002 (now U.S. Pat. No. 6,792,591),
which is a continuation in part of application Ser. No. 09/840,305
filed on Apr. 24, 2001 (now U.S. Pat. No. 6,519,760).
Claims
We claim:
1. A method of generating a mask by identifying undesirable pitches
between features when designing an integrated device to be formed
on a substrate by use of a lithographic apparatus utilizing said
mask, said method comprising the steps of: (a) identifying extreme
interaction pitch regions by determining illumination intensity
levels for a given illumination angle over a range of pitches; (b)
identifying said undesirable pitches for each extreme interaction
pitch region identified in step (a) by determining illumination
intensities for a given extreme interaction pitch region over a
range of illumination angles; (c) generating a mask pattern by
arranging features such that no combination of features in said
mask pattern has a pitch in an extreme interaction pitch region for
which illumination angles in said desired illumination scheme are
undesirable; and (d) positioning scattering bars in said mask
pattern based on said extreme interaction pitch regions.
2. A method according to claim 1, wherein said extreme interaction
pitch regions define regions which exhibit either substantial
constructive optical interference or substantial destructive
optical interference.
3. A method according to claim 1, wherein said undesirable pitches
have corresponding illumination intensities exceeding a
predetermined value.
4. A method according to claim 1, wherein said scattering bars are
non-resolvable features operative for providing optical proximity
correction.
5. A computer program product for controlling a computer comprising
a recording medium readable by the computer, means recorded on the
recording medium for directing the computer to generate files
corresponding to a mask for use in an lithographic imaging process,
said generation of said files comprising the steps of: (a)
identifying extreme interaction pitch regions by determining
illumination intensity levels for a given illumination angle over a
range of pitches; (b) identifying said undesirable pitches for each
extreme interaction pitch region identified in step (a) by
determining illumination intensities for a given extreme
interaction pitch region over a range of illumination angles; and
(c) positioning scattering bars in said mask based on said extreme
interaction pitch regions.
6. The computer product of claim 5, wherein said extreme
interaction pitch regions define regions which exhibit either
substantial constructive optical interference or substantial
destructive optical interference.
7. The computer product of claim 5, wherein said undesirable
pitches have corresponding illumination intensities exceeding a
predetermined value.
8. The computer product of claim 5, further comprising the step of
designing a mask pattern by arranging features such that no
combination of features in said mask pattern has a pitch in an
extreme interaction pitch region for which illumination angles in
said desired illumination scheme are undesirable.
9. The computer product of claim 5, wherein said scattering bars
are non-resolvable features operative for providing optical
proximity correction.
Description
FIELD OF THE INVENTION
The present invention relates to photolithography and more
particularly to optical proximity correction methods used during
the development of photolithography masks for use in lithographic
apparatus comprising: a radiation system for supplying a projection
beam of radiation; a support structure for supporting patterning
means, the patterning means serving to pattern the projection beam
according to a desired pattern; a substrate table for holding a
substrate; and a projection system for projecting the patterned
beam onto a target portion of the substrate.
BACKGROUND OF THE INVENTION
The term "patterning means" as here employed should be broadly
interpreted as referring to means that can be used to endow an
incoming radiation beam with a patterned cross-section,
corresponding to a pattern that is to be created in a target
portion of the substrate; the term "light valve" can also be used
in this context. Generally, the said pattern will correspond to a
particular functional layer in a device being created in the target
portion, such as an integrated circuit or other device (see below).
Examples of such patterning means include: A mask. The concept of a
mask is well known in lithography, and it includes mask types such
as binary, alternating phase-shift, and attenuated phase-shift, as
well as various hybrid mask types. Placement of such a mask in the
radiation beam causes selective transmission (in the case of a
transmissive mask) or reflection (in the case of a reflective mask)
of the radiation impinging on the mask, according to the pattern on
the mask. In the case of a mask, the support structure will
generally be a mask table, which ensures that the mask can be held
at a desired position in the incoming radiation beam, and that it
can be moved relative to the beam if so desired. A programmable
mirror array. One example of such a device is a matrix-addressable
surface having a viscoelastic control layer and a reflective
surface. The basic principle behind such an apparatus is that (for
example) addressed areas of the reflective surface reflect incident
light as diffracted light, whereas unaddressed areas reflect
incident light as undiffracted light. Using an appropriate filter,
the said undiffracted light can be filtered out of the reflected
beam, leaving only the diffracted light behind; in this manner, the
beam becomes patterned according to the addressing pattern of the
matrix-addressable surface. An alternative embodiment of a
programmable mirror array employs a matrix arrangement of tiny
mirrors, each of which can be individually tilted about an axis by
applying a suitable localized electric field, or by employing
piezoelectric actuation means. Once again, the mirrors are
matrix-addressable, such that addressed mirrors will reflect an
incoming radiation beam in a different direction to unaddressed
mirrors; in this manner, the reflected beam is patterned according
to the addressing pattern of the matrix-addressable mirrors. The
required matrix addressing can be performed using suitable
electronic means. In both of the situations described hereabove,
the patterning means can comprise one or more programmable mirror
arrays. More information on mirror arrays as here referred to can
be gleaned, for example, from U.S. Pat. Nos. 5,296,891 and
5,523,193, and PCT patent applications WO 98/38597 and WO 98/33096,
which are incorporated herein by reference. In the case of a
programmable mirror array, the said support structure may be
embodied as a frame or table, for example, which may be fixed or
movable as required. A programmable LCD array. An example of such a
construction is given in U.S. Pat. No. 5,229,872, which is
incorporated herein by reference. As above, the support structure
in this case may be embodied as a frame or table, for example,
which may be fixed or movable as required.
For purposes of simplicity, the rest of this text may, at certain
locations, specifically direct itself to examples involving a mask
and mask table; however, the general principles discussed in such
instances should be seen in the broader context of the patterning
means as hereabove set forth.
Lithographic projection apparatus can be used, for example, in the
manufacture of integrated circuits (ICs). In such a case, the
patterning means may generate a circuit pattern corresponding to an
individual layer of the IC, and this pattern can be imaged onto a
target portion (e.g. comprising one or more dies) on a substrate
(silicon wafer) that has been coated with a layer of
radiation-sensitive material (resist). In general, a single wafer
will contain a whole network of adjacent target portions that are
successively irradiated via the projection system, one at a time.
In current apparatus, employing patterning by a mask on a mask
table, a distinction can be made between two different types of
machine. In one type of lithographic projection apparatus, each
target portion is irradiated by exposing the entire mask pattern
onto the target portion in one go; such an apparatus is commonly
referred to as a wafer stepper. In an alternative
apparatus--commonly referred to as a step-and-scan apparatus--each
target portion is irradiated by progressively scanning the mask
pattern under the projection beam in a given reference direction
(the "scanning" direction) while synchronously scanning the
substrate table parallel or anti-parallel to this direction; since,
in general, the projection system will have a magnification factor
M (generally <1), the speed V at which the substrate table is
scanned will be a factor M times that at which the mask table is
scanned. More information with regard to lithographic devices as
here described can be gleaned, for example, from U.S. Pat. No.
6,046,792, incorporated herein by reference.
In a manufacturing process using a lithographic projection
apparatus, a pattern (e.g. in a mask) is imaged onto a substrate
that is at least partially covered by a layer of
radiation-sensitive material (resist). Prior to this imaging step,
the substrate may undergo various procedures, such as priming,
resist coating and a soft bake. After exposure, the substrate may
be subjected to other procedures, such as a post-exposure bake
(PEB), development, a hard bake and measurement/inspection of the
imaged features. This array of procedures is used as a basis to
pattern an individual layer of a device, e.g. an IC. Such a
patterned layer may then undergo various processes such as etching,
ion-implantation (doping), metallization, oxidation,
chemo-mechanical polishing, etc., all intended to finish off an
individual layer. If several layers are required, then the whole
procedure, or a variant thereof, will have to be repeated for each
new layer. Eventually, an array of devices will be present on the
substrate (wafer). These devices are then separated from one
another by a technique such as dicing or sawing, whence the
individual devices can be mounted on a carrier, connected to pins,
etc. Further information regarding such processes can be obtained,
for example, from the book "Microchip Fabrication: A Practical
Guide to Semiconductor Processing", Third Edition, by Peter van
Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4,
incorporated herein by reference.
For the sake of simplicity, the projection system may hereinafter
be referred to as the "lens"; however, this term should be broadly
interpreted as encompassing various types of projection system,
including refractive optics, reflective optics, and catadioptric
systems, for example. The radiation system may also include
components operating according to any of these design types for
directing, shaping or controlling the projection beam of radiation,
and such components may also be referred to below, collectively or
singularly, as a "lens". Further, the lithographic apparatus may be
of a type having two or more substrate tables (and/or two or more
mask tables). In such "multiple stage" devices the additional
tables may be used in parallel, or preparatory steps may be carried
out on one or more tables while one or more other tables are being
used for exposures. Dual stage lithographic apparatus are
described, for example, in U.S. Pat. No. 5,969,441 and WO 98/40791,
incorporated herein by reference.
As semiconductor manufacturing technology is quickly pushing
towards the limits of optical lithography, the state-of-the-art
processes to date have regularly produced ICs with features
exhibiting critical dimensions ("CDs") which are below the exposure
wavelength (".lamda."). A "critical dimension" of a circuit is
defined as the smallest width of a feature or the smallest space
between two features. For feature patterns that are designed to be
smaller than .lamda., it has been recognized that the optical
proximity effect (OPE) becomes much more severe, and in fact
becomes intolerable for leading edge sub-.lamda. production
processes.
Optical proximity effects are a well known characteristic of
optical projection exposure tools. More specifically, proximity
effects occur when very closely spaced circuit patterns are
lithographically transferred to a resist layer on a wafer. The
light waves of the closely spaced circuit features interact,
thereby distorting the final transferred pattern features. In other
words, diffraction causes adjacent features to interact with each
other in such a way as to produce pattern dependent variations. The
magnitude of the OPE on a given feature depends on the feature's
placement on the mask with respect to other features.
One of the primary problems caused by such proximity effects is an
undesirable variation in feature CDs. For any leading edge
semiconductor process, achieving tight control over the CDs of the
features (i.e., circuit elements and interconnects) is typically
the primary manufacturing goal, since this has a direct impact on
wafer sort yield and speed-binning of the final product.
It has been known that the variations in the CDs of circuit
features caused by OPE can be reduced by several methods. One such
technique involves adjusting the illumination characteristics of
the exposure tool. More specifically, by carefully selecting the
ratio of the numerical aperture of the illumination condenser
("NAc") to the numerical aperture of the imaging objective lens
("NAo") (this ratio has been referred to as the partial coherence
ratio--.sigma.), the degree of OPE can be manipulated to some
extent.
In addition to using relatively incoherent illumination, such as
described above, OPE can also be compensated for by
"pre-correcting" the mask features. This family of techniques is
generally known as optical proximity correction (OPC)
techniques.
For example, in U.S. Pat. No. 5,242,770 (the '770 patent), which is
hereby incorporated by reference, the method of using scattering
bars (SBs) for OPC is described. The '770 patent demonstrates that
the SB method is very effective for modifying isolated features so
that the features behave as if the features are dense features. In
so doing, the depth of focus (DOF) for the isolated features is
also improved, thereby significantly increasing process latitude.
Scattering bars (also known as intensity leveling bars or assist
bars) are correction features (typically non-resolvable by the
exposure tool) that are placed next to isolated feature edges on a
mask in order to adjust the edge intensity gradients of the
isolated edges. Preferably, the adjusted edge intensity gradients
of the isolated edges match the edge intensity gradients of the
dense feature edges, thereby causing the SB-assisted isolated
features to have nearly the same width as densely nested
features.
It is generally understood that the process latitude associated
with dense structures is better than that associated with isolated
structures under conventional illumination for large feature sizes.
However, recently, more aggressive illumination schemes such as
annular illumination and multipole illumination have been
implemented as a means of improving resolution and known OPC
techniques have not always had the desired effects with such
illumination schemes.
SUMMARY OF THE INVENTION
An object of the invention is to provide a method for optimizing
mask patterns for use with various different illumination
schemes.
Accordingly, the present invention provides a method and technique
for identifying and eliminating forbidden pitch regions, which
degrade the overall printing performance, so as to allow for an
improvement of the CDs and process latitude obtainable utilizing
currently known photolithography tools and techniques. The
"forbidden pitch" regions are regions in which both the critical
dimension of the feature and the process latitude of the feature
are negatively affected.
When utilizing such illumination schemes, the inventors of the
present invention have noted that some optical phenomenon have
become more prominent. In particular, the inventors have noticed a
forbidden pitch phenomena. More specifically, there are pitch
ranges within which the process latitude of a "densely located"
main feature, especially the exposure latitude, is worse than that
of an isolated feature of the same size. This important observation
indicates that the existence of the neighboring feature is not
always beneficial for main feature printing, which is in
contradiction to what is commonly conceived, prior to the discovery
by the present inventors. Indeed, the present inventors believe
that the forbidden pitch phenomenon has become a limiting factor in
advanced photolithography. As such, suppressing the forbidden pitch
phenomenon will be necessary to further improve the CDs and process
latitude obtainable utilizing currently known semiconductor device
manufacturing tools and techniques.
More specifically, the present invention relates to a method of
identifying undesirable pitches between features when designing an
integrated circuit (or other device) to be formed on a substrate by
use of a lithographic exposure tool. In an exemplary embodiment,
the method comprises the steps of (a) identifying extreme
interaction pitch regions by determining illumination intensity
levels for a given illumination angle over a range of pitches; and
(b) identifying the undesirable pitches for each extreme
interaction pitch region identified in step (a) by determining
illumination intensities for a given extreme interaction pitch
region over a range of illumination angles.
In accordance with the present invention, it is shown that the
variation of the critical dimension as well as the process latitude
of a main feature is a direct consequence of light field
interference between the main feature and the neighboring features.
Depending on the phase of the field produced by the neighboring
features, the main feature critical dimension and process latitude
can be improved by constructive light field interference, or
degraded by destructive light field interference. The phase of the
field produced by the neighboring features can be shown to be
dependent on the pitch as well as the illumination angle. For a
given illumination angle, the forbidden pitch lies in the location
where the field produced by the neighboring features interferes
with the field of the main feature destructively. The present
invention provides a method for identifying the forbidden pitch
regions (i.e., locations) for any feature size and any illumination
condition. More importantly, the present invention provides a
method for performing illumination design in order to suppress the
forbidden pitch phenomena, thereby suppressing the negative effects
associated therewith. In addition, the present invention provides
for a method for utilizing scattering bar placement in conjunction
with the suppression of the forbidden pitch phenomena to further
minimize optical proximity effects and optimize overall printing
performance.
As described in further detail below, the present invention
provides significant advantages over the prior art. Most
importantly, the present invention provides for identifying and
eliminating forbidden pitch regions, which degrade the overall
printing performance, thereby allowing for an improvement of the
CDs and process latitude obtainable utilizing currently known
photolithography tools and techniques.
It will be appreciated that in the present invention, the "mask
pattern" may be embodied in a mask but may also be applied using
another type of patterning means, examples of which are mentioned
above. The term "mask pattern" is used herein for convenience but
should not be construed as requiring the presence of a mask, unless
the context otherwise requires.
Although specific reference may be made in this text to the use of
the apparatus according to the invention in the manufacture of ICs,
it should be explicitly understood that such an apparatus has many
other possible applications. For example, it may be employed in the
manufacture of integrated optical systems, guidance and detection
patterns for magnetic domain memories, liquid-crystal display
panels, thin-film magnetic heads, etc. The skilled artisan will
appreciate that, in the context of such alternative applications,
any use of the terms "reticle", "wafer" or "die" in this text
should be considered as being replaced by the more general terms
"mask", "substrate" and "target portion", respectively.
In the present document, the terms "radiation" and "beam" are used
to encompass all types of electromagnetic radiation, including
ultraviolet radiation (e.g. with a wavelength of 365, 248, 193, 157
or 126 nm) and EUV (extreme ultra-violet radiation, e.g. having a
wavelength in the range 5 20 nm), as well as particle beams, such
as ion beams or electron beams.
Additional advantages of the present invention will become apparent
to those skilled in the art from the following detailed description
of exemplary embodiments of the present invention.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention itself, together with further objects and advantages,
can be better understood by reference to the following detailed
description and the accompanying drawings, in which:
FIG. 1a illustrates an exemplary imaging system.
FIGS. 1b and 1c illustrate the transformation of an on-axis image
point at the exit pupil into a corresponding point in the
two-dimensional frequency plane.
FIG. 2 represents an exemplary mask pattern to be printed on a
wafer.
FIGS. 3a 3d illustrate exemplary results of both on-axis
illumination and off-axis illumination.
FIGS. 4a and 4b illustrate an exemplary interaction between side
features and the main feature at two different pitches under a
specific illumination angle.
FIG. 5 is a graph representing two-dimensional illumination.
FIG. 6 is a graph representing the conjugated illumination scheme
required for vertical and horizontal feature performance
balance.
FIG. 7 illustrates a flow chart detailing the process of
defining/identifying the forbidden pitch regions.
FIG. 8 is a plot resulting from the process of FIG. 7, which
illustrates the extreme interaction pitch regions.
FIG. 9 illustrates a flow chart detailing the process of generating
the illumination map for a given pitch.
FIGS. 10a 10c show the illumination maps corresponding to the
extreme interaction pitch regions of 480 nm, 560 nm, 635 nm
illustrated in FIG. 8, respectively.
FIG. 10d illustrates the illumination map corresponding to the
pitch of 310 nm.
FIG. 11 illustrates an illumination design that improves the
exposure latitude at the 480 nm pitch region while preserving
strong constructive structural interactions at other pitch
regions.
FIG. 12 is a graph illustrating a comparison of the log-slope
values associated with annular, quadrupole and the modified
quadrupole illumination.
FIG. 13 shows the extreme interaction edge-to-edge placement
positions of the scattering bars around an isolated main line.
FIGS. 14a d are illumination maps for scattering bars having
varying separation from a main feature.
FIG. 15 schematically depicts a lithographic projection apparatus
suitable for use with a mask designed with the aid of the current
invention.
DETAILED DESCRIPTION OF THE INVENTION
As explained in more detail below, the forbidden pitch phenomenon
is a direct consequence of optical interactions between neighboring
features. More specifically, the field phase of the neighboring
feature relative to that of the main feature depends on the
illumination angle and the separation distance between the
features. For a given illumination angle, there are pitch ranges
within which the field phase produced by the neighboring feature is
substantially 180.degree. out of phase relative to the field phase
of the main feature, thereby resulting in destructive interference.
Such destructive interference reduces the image contrast of the
main feature, and as a result, causes a loss of exposure latitude.
It is these pitch ranges, which cause destructive interference,
that are referred to as the forbidden pitch ranges, and that are
identified and eliminated by the methods of the present
invention.
In accordance with the methods of the present invention, and as
explained in detail below, the forbidden pitch regions (i.e., the
extreme structural interaction pitch regions) are mapped out or
identified utilizing an illumination map. In one embodiment, for
each extreme structural interaction pitch, a corresponding
illumination map is obtained, which shows its favorable
illumination regions and its unfavorable illumination regions. As
such, by utilizing the illumination maps the undesirable forbidden
pitch regions can be eliminated. Furthermore, when the neighboring
feature size is changed to the scattering bar size, similar
constructive and destructive interference regions can be located
and their corresponding illumination maps can also be obtained.
Based on these illumination maps, the present invention also allows
for optimal scattering bar placement to be determined for a given
illumination condition.
Prior to discussing the details of the present invention, a brief
review of the theory relevant to the method of the present
invention is presented. In accordance with Fourier optics, the
imaging process can be viewed as a double diffraction process under
coherent illumination. The lens acts as the Fourier transform
device that converts the geometrical information of the object
(i.e., the reticle) into the spatial frequency information of the
object in the frequency domain. The spatial frequency information
of the object (i.e., frequency components and their amplitudes) is
displayed at the exit pupil of the optical imaging system. If the
linear dimensions of the geometrical figures on the object are much
larger than the illumination wavelength, and the topology of the
object is much smaller than the illumination wavelength, then the
object can be viewed as purely geometrical and scalar diffraction
theory is applicable.
The foregoing assumptions are currently considered valid for a
reduction projection optical imaging system with a binary chrome
reticle. In such cases, the electric field at the exit pupil is
related to the transmission function of the object through the
Fourier transformation. Although 4.times. or 5.times. reduction
projection systems are utilized in practical photolithography
systems, the following discussion utilizes a 1.times. system in
order to simplify the analysis. The 1.times. optical imaging system
avoids the complexity of information conversion from entrance pupil
to exit pupil that is required for a 4.times. or 5.times. reduction
optical imaging system, namely, the spatial frequency conversion,
the field magnitude conversion and polarization tracking. It is
noted, however, that the present invention is equally applicable to
other systems, including a 4.times. or 5.times. reduction optical
imaging system, or any other applicable system.
FIG. 1a illustrates an exemplary imaging system 10 helpful for
describing the operation of the present invention. As shown, the
imaging system 10 comprises a monochromatic light source 12, a
condenser 14, a reticle 16, and a projection lens 18. As also
shown, the imaging process generates an exit pupil 20 and an image
plane 22. In the given system, the illumination scheme is Kohler
illumination so that uniform illumination is achieved. Furthermore,
if an adjustable aperture stop is placed at the back focal plane of
the projection lens 18, then the back focal plane becomes the exit
pupil because there are no optical elements between the back focal
plane and the image plane. When examining the exit pupil from the
on-axis image point, each geometrical point at the exit pupil 20
corresponds to a pair of angular coordinates (.theta.,.phi.), which
can be transformed into a corresponding point in the
two-dimensional frequency plane through the following
transformation, described by equation (1) and shown in FIGS. 1b and
1c. k.sub.x=sin .theta. cos .phi., k.sub.y=sin .theta. sin .phi.
(1)
Now considering an object with a transmission function as shown in
FIG. 2, such an object can be treated as a one-dimensional object,
with its transmission function expressed as,
.function..times..alpha..times..times..times..function..times..function..-
times..function.<> ##EQU00001## a is the width of the main
feature (the center feature), c is the width of the side feature(s)
and b is the edge-to-edge separation distance between the main
feature and the side feature. The object illustrated in FIG. 2
represents a generalized mask pattern. When .alpha.=0, it is a
binary mask, .alpha.=sqrt (0.06)=0.24 for a 6% attenuated phase
shift mask, and .alpha.=1.0 for a chrome-less phase shift mask.
Under on-axis coherent illumination (sin .theta.=0) with
quasi-monochromatic light source, the field at the exit pupil
is
.function..varies..lamda..times..intg..infin..infin..times..function..tim-
es.e.times..pi..times..times..times..lamda..times.d ##EQU00002##
where k.sub.x=sin .theta. is the spatial frequency in the frequency
plane along the k.sub.x axis. It is noted that by a
quasi-monochromatic light source, it is meant that the coherence
length of the light is much longer than the optical path difference
between any pair of light rays under consideration. This
approximation holds well for light sources used in
photolithography, especially the KrF excimer light source with its
bandwidth less than 1.0 picometer.
FIGS. 3a 3d illustrate the results of both on-axis illumination and
off-axis illumination along the x-axis. As shown in FIGS. 3a and
3b, under on-axis illumination, the spectrum of the object is
centered. However, under off-axis illumination along the x-axis, as
shown in FIGS. 3c and 3d, the spectrum of the object is shifted
relative to the exit pupil, and the field at the exit pupil
becomes,
.function..varies..lamda..times..intg..infin..infin..times..function..tim-
es.e.times..pi..times..times..lamda..times.e.times..pi..times..times..lamd-
a..times.d ##EQU00003## where k.sub.xo=sin .theta..sub.o and
.theta..sub.o is the illumination angle. The phase term
e.sup.2.pi.ikxoxo/.lamda. has a simple geometrical interpretation,
and accounts for the phase difference of the illumination field at
different object points, as illustrated in FIG. 3c. By inserting
equation (2) into equation (5), the result is:
.function..varies..delta..function..alpha..times..lamda..times..function.-
.pi..times..lamda..times..pi..times..lamda..times..times.e.times..pi..time-
s..times..function..lamda..times..lamda..times..function..pi..times..lamda-
..times..pi..times..lamda..times..times.e.times..pi..times..times..functio-
n..lamda..times..lamda..times..function..pi..times..lamda..times..pi..time-
s..lamda..times. ##EQU00004## where p=b+a/2+c/2 is defined as the
pitch of the pattern. The electric field at the image plane,
according to Fourier optics, is
.function..varies..lamda..times..intg..times..function..times.e.times..pi-
..times..times..lamda..times.d ##EQU00005## It is noted that the
quantities set forth in equations (6) and (7) can be rescaled such
that all the geometrical dimensions are normalized to .lamda./NA,
and k.sub.x and k.sub.xo are normalized to NA. Explicitly, these
rescaled quantities can be expressed as, a.sub.r=aNA/.lamda.,
b.sub.r=bNA/.lamda., c.sub.r=cNA/.lamda., p.sub.r=pNA/.lamda. (8)
x.sub.i.sup.r=x.sub.iNA/.lamda., k.sub.x.sup.r=k.sub.x/NA,
k.sub.xo.sup.r=k.sub.xo/NA=s Using these rescaled quantities, the
electric field at the image plane becomes:
.times..varies.e.times..pi..times..times..times..times..alpha..times..int-
g..times..times..function..pi..times..times..function..pi..times..times..f-
unction..times.e.times..times..pi..times..times..function..times..times..f-
unction..pi..times..times..function..pi..times..times..function..times.e.t-
imes..times..pi..times..times..function..times..times..function..pi..times-
..times..function..pi..times..times..function..times.e.times..pi..times..t-
imes..times..times.d.times..times..function..varies.e.times..pi..times..ti-
mes..times..times..alpha..times..intg..times..times..function..pi..times..-
times..function..pi..times..times..function..times.e.times..pi..times..tim-
es..times..times..times..times..times..alpha..times.e.times..pi..times..ti-
mes..times..intg..times..times..function..pi..times..times..function..pi..-
times..times..function..times.e.times..times..pi..times..times..function..-
times..times.d.times..alpha..times.e.times..times..pi..times..times..times-
..times..intg..times..times..function..pi..times..times..function..pi..tim-
es..times..function..times.e.times..times..pi..times..times..function..tim-
es..times.d' ##EQU00006## where s is related to the illumination
angle. From equation (9'), it is clear that the fields produced by
the side features have a phase term e.sup..+-.2.pi.ip.sup.r.sup.s.
It is this phase term that plays the central role in the
determination and elimination of the forbidden pitch regions.
It is noted that equation (9) or (9') applies for one-dimensional
illumination. However, as shown below, the two-dimensional
illumination used in photolithography can be approximated as a
one-dimensional illumination for long lines or trench
structures.
As detailed above, it has been determined that under certain
illumination conditions there are some pitch regions within which
the exposure latitudes of the main feature become very small, even
smaller than that of the isolated feature. Such pitch regions are
referred to as forbidden pitch regions, and are caused by
destructive interaction between the main feature and the side
features under those illumination conditions. Whether the existence
of the side features will improve the process latitude of the main
feature or degrade the process latitude of the main feature depends
on the fields produced by these side features at the main feature
Gaussian image point. If the fields of the side features have the
same phases as the field of the main feature at the main feature
image location, then constructive interference between these fields
can improve the process latitude of the main feature. If the fields
of the side features have 180.degree. phase difference with respect
to the field of the main feature at the main feature image
location, then destructive interference between the fields causes
degrading of the process latitude for the main feature. The
forbidden pitch regions lie in the locations where destructive
interference occurs under a given illumination condition. When such
situations arise, the process latitude of the main feature is worse
than that of the isolated feature. Since the field signs (depending
on phases) and their magnitudes from the side features are
determined by the pitch, the illumination angle (s) and the
numerical aperture (NA), constructive and destructive interaction
pitch regions can be located using equation (9'). FIGS. 4a and 4b
show examples of the interaction between side features and the main
feature at two different pitches under a specific illumination
angle. In the given examples, the feature size is 130 nm, NA=0.65,
and s=0.4 for a binary mask (.alpha.=0). As illustrated in FIG. 4a,
at a pitch of approximately 470 nm, the minimum intensity of the
main feature (dashed line) at its Gaussian image point is higher
than that of an isolated feature (solid line), leading to a lower
image contrast and smaller exposure latitude. As shown in FIG. 4b,
at a pitch of approximately 680 nm, the minimum intensity of the
main feature (dashed line) at its Gaussian image point is lower
than that of an isolated feature (solid line), leading to a higher
image contrast and larger exposure latitude.
As noted above, the foregoing analysis of forbidden pitch regions
is based on one-dimensional illumination, i.e. (k.sub.y=0). In
actuality, the illumination schemes implemented in photolithography
are two-dimensional. However, for structures that can be
approximated as one-dimensional, such as very long lines or
trenches, the two-dimensional illumination problem can be reduced
to a one-dimensional problem. The foregoing is illustrated
utilizing FIG. 5. Referring to FIG. 5, assuming the structures are
infinitely long in the y direction, the Fourier transform spectrum
of the structure at the exit pupil will have zero width in the
k.sub.y direction. Under this scenario, a two-dimensional
illumination (NA, k.sub.x, k.sub.y) is equivalent to a
one-dimensional illumination (NA.sub.effective, s.sub.effective).
The relationship between the two-dimensional illumination and its
corresponding one-dimensional illumination is readily derived,
.times..beta..times..times..alpha..beta..beta.< ##EQU00007##
where NA in equation (10) is the numerical aperture setting in the
employed lithographic projection apparatus.
Further detailed analysis on the forbidden pitch phenomenon and
optimal illumination design for suppressing forbidden pitch regions
has to take into account the performance balance between "vertical"
and "horizontal" features (i.e., features in the y and x
direction). To achieve this performance balance, an illumination
source point (.alpha., .beta.) in the (k.sub.x>0, k.sub.y>0)
illumination space must have a corresponding conjugated
illumination source point (-.beta., .alpha.) in the (k.sub.x<0,
k.sub.y>0) illumination space, as shown in FIG. 6. Such
performance balance between "vertical" and "horizontal" features is
required for single exposure schemes, which is widely used today.
However, in multiple exposure schemes such as dipole illumination a
double exposure scheme conjugated illumination source point is not
required. Nevertheless, the theory and methodology outlined below
can also be applied to multiple exposure schemes with some
modifications based on the illumination scheme employed. For a
single exposure scheme, each illumination point in the first
quadrant exhibits a 90 degree rotational symmetry with a
corresponding illumination point in the second quadrant. In other
words, each illumination point in the first quadrant exhibits a 90
degree rotational symmetry with a corresponding illumination point
in the second quadrant. Similarly, in the reduced one-dimensional
illumination space, any illumination source point
(NA.sub.effective, S.sub.effective) must have a conjugated
illumination point ( {square root over
(NA.sup.2-NA.sub.effective.sup.2S.sub.effective.sup.2)},- {square
root over (NA.sup.2-NA.sub.effective.sup.2)}/ {square root over
(NA.sup.2-NA.sub.effective.sup.2S.sub.effective.sup.2)}).
Utilizing this conjugated illumination scheme, the forbidden pitch
regions can be identified and eliminated. FIG. 7 illustrates a flow
chart detailing the process of defining/identifying the forbidden
pitch regions. The first portion of the process entails determining
the interaction pitch regions for a given (.alpha., .beta.).
Referring to FIG. 7, this is accomplished by utilizing equation 9
or 9', which as explained above represents the calculation engine
associated with one-dimensional illumination. More specifically,
for a given illumination point (i.e., .alpha., .beta. are fixed),
equation 9 or 9' is utilized to calculate the illumination
intensity at a given pitch, I(.alpha., .beta., pitch) (Step 70). In
addition, equation 9 or 9' is utilized to calculate the
illumination intensity of the corresponding 90 degree rotational
symmetric point at the same pitch, I(-.beta., .alpha., pitch) (Step
72). The two illumination intensities are then added together (Step
74) to obtain I.sub.total (.alpha., .beta., pitch), and then the
log-slope of I.sub.total is calculated (Step 76). This process is
then repeated for each pitch of interest, I(.alpha., .beta., pitch)
(Steps 78, 80).
FIG. 8 illustrates a plot of the results of the process of FIG. 7,
which depicts the areas having extreme interaction pitch regions.
Referring to FIG. 8, the extreme pitch interaction locations are
identified by those areas having a substantial amount of circles.
More specifically, the extreme pitch interaction locations can be
identified utilizing the following equation: d(log-slope of
I.sub.total)/d(pitch).about.0. (11) In particular, the locations
substantially proximate the location satisfying the foregoing
equation are extreme pitch interaction locations. In other words,
while the foregoing condition for locating the extreme pitch
interaction locations specifies a specific location, the actual
forbidden pitch is a range around this location. The actual range
is dependent on the wavelength and the NA of the exposure
apparatus. From experimental studies, it was determined that the
forbidden pitch range around a given specific location is
approximately +/-0.12 wavelength/NA. For example, if exposure
apparatus utilizes a 248 nm source and a NA=0.65, then the extreme
interaction pitch range is approximately +/-45 nm. It is further
noted that while the extreme interaction pitch locations are
relatively stable, they are not stationary. Extreme interaction
pitch locations can shift slightly with variations in illumination
angle.
Returning to FIG. 8, it is noted that the example set forth in FIG.
8 was conducted utilizing a set feature size of 130 nm, a scanner
NA=0.65, and s effective=0.65. As shown, there are four distinct
extreme interaction pitch regions in the intermediate pitch range
(300 nm to 700 nm), which are located at approximately 370 nm, 480
nm, 560 nm and 630 nm. It is further noted that FIG. 8 does not
indicate whether the extreme interaction pitch regions are
constructive or destructive, but just whether or not such regions
exist. Also, typically the extreme interaction pitch regions do not
vary appreciably with the illumination angle. The regions tend not
to be very sensitive to illumination angle.
Once the extreme interaction pitch regions are identified, the next
portion of the process entails generating illumination maps for the
pitches of interest/concern (i.e., the extreme interaction pitch
regions). To summarize, for each extreme interaction pitch region,
the log-slope of the main feature image at the mask edge is
calculated as a function of illumination angle. FIG. 9 illustrates
a flow chart detailing the process of generating the illumination
map for a given extreme interaction pitch.
Referring to FIG. 9, again utilizing equation 9 or 9', the
illumination intensity for a first illumination angle (.alpha.,
.beta.) and the fixed pitch is calculated (Step 90), and the
illumination intensity of the corresponding 90 degree rotational
symmetric point at the same pitch is calculated (Step 92). The two
illumination intensities are then added together (Step 94) to
obtain I.sub.total (.alpha., .beta., pitch), and then the log-slope
of I.sub.total is calculated (Step 96). This process is then
repeated for a plurality of illumination angles so as to allow the
illumination map to cover at least one quadrant (i.e.,
0.ltoreq.k.sub.x.ltoreq.1, 0.ltoreq.k.sub.y.ltoreq.1), (Steps 98,
100). FIGS. 10a 10c show the illumination maps corresponding to the
extreme interaction pitch regions of 480 nm, 560 nm, 635 nm
illustrated in FIG. 8, respectively. FIG. 10d illustrates the
illumination map corresponding to the minimum pitch of 310 nm.
Referring again to FIGS. 10a 10d, the illumination angles
corresponding to higher values of the log-slope of I.sub.total are
the illumination angles that provide optimal performance for the
given pitch. In other words, the higher the value of the log-slope
of I.sub.total, the better the performance. For example, referring
to FIG. 10a, the optimal illumination angle for this pitch (i.e.,
480 nm) is approximately zero. Any illumination angle corresponding
to values of k.sub.x>0.2 and k.sub.y>0.2 result in low values
of the log-slope of I.sub.total, and are therefore undesirable. As
shown in FIG. 10a, the highest values of the log-slope occur when
both k.sub.x and k.sub.y are approximately zero. Referring to FIG.
10b, the optimal illumination angles for the 560 nm pitch are
angles corresponding approximately to either k.sub.x=0.5 and
k.sub.y=0, or k.sub.x=0 and k.sub.y=0.5.Referring to FIG. 10c, the
optimal illumination angles for the 635 nm pitch are angles
corresponding approximately to k.sub.x=0.3 and k.sub.y=0.3.Finally,
referring to FIG. 10d, the optimal illumination angles for the 310
nm pitch are angles corresponding to either k.sub.x=0.5 and
k.sub.y=0.5.
Accordingly, from the illumination maps, it is clear that whether
an extreme interaction pitch becomes a forbidden pitch region or a
friendly pitch region depends on the illumination employed. Further
examination of the illumination maps reveals that the illumination
map at pitch 480 nm is complementary to the illumination maps at
635 nm and 310 nm. More specifically, at pitch 480 nm, the
desirable illumination angles correspond to k.sub.x and k.sub.y
equal to approximately zero, and the undesirable areas correspond
to k.sub.x and k.sub.y equal to approximately 0.5. Conversely, at
pitches of 635 nm and 310 nm, the desirable illumination angles
correspond to k.sub.x and k.sub.y equal to approximately 0.5, and
the undesirable areas correspond to k.sub.x and k.sub.y equal to
approximately 0. This intrinsic complementary property prevents
taking advantage of the quadrupole illumination for 130 nm mode
photolithography unless there are no structures around 480 nm pitch
on the layer. Analysis of the foregoing illumination maps allows
the designer to select the illumination angle(s) to be utilized so
as to optimize the printing performance, and more importantly, to
avoid the extreme interaction pitch areas which result in
destructive interference.
It is noted that the minimum value of the log-slope of I.sub.total
associated with acceptable performance depends in-part on the
resist being utilized. For example, different resists exhibit
different contrasts, which require different minimum values of the
log-slope of I.sub.total corresponding to optimal performance
regions. As a general rule, however, a value of the log-slope of
I.sub.total approximately equal to a greater than 15 results in an
acceptable process.
With regard to optimizing printing performance, referring to the
exemplary illumination maps set forth in FIGS. 10a 10d, it is noted
that in comparison with quadrupole illumination, annular
illumination (.sigma._in=0.55 and .sigma._out=0.85, for example)
can improve image contrast around the 480 nm pitch region by
degrading the image contrast at other pitch regions. Such an
approach reduces the structural interactions at different pitches
by averaging the constructive and destructive interactions within
the illumination space.
For example, FIG. 11 illustrates an illumination design that
improves the exposure latitude at the 480 nm pitch region while
preserving strong constructive structural interactions at other
pitch regions. More specifically, FIG. 11 illustrates an
illumination design that provides some illumination at the
illumination center since the favorable illumination for the 480 nm
pitch region is around center (k.sub.x=0, k.sub.y=0). However, the
performance balance has to be considered when illumination at
center is added, because the illumination at the center will
unavoidably degrade the image contrast at the minimum pitch region
at 310 nm. Comparison of the log-slope using Solid-C simulation
software on annular, quadrupole and the modified quadrupole
illumination (.sigma._center=0.15 and .sigma._center=0.2) is shown
in FIG. 12. The features are 130 nm lines on a 6% attenuated phase
shift mask. As shown from the simulation results, the modified
quadrupole with center .sigma.=0.2 will provide an overall better
process, and it also allows taking full advantage of the assist
feature benefits.
It is noted that the term QUASAR used in the figures refers to the
generation of quadrupole illumination using a Diffractive Optical
Element (DOE), which re-distributes incoming radiation flux rather
than blocking/passing it. In particular, 30-degree QUASAR refers to
a quadrupole pattern in which the 4 poles are segments of an
annulus, and each subtends an angle of 30 degrees with the center
of the annulus.
It is also possible to utilize the foregoing illumination maps to
assist in the placement of scattering bars, which operate to
mitigate optical proximity effects. The use of such scattering bars
has been described in U.S. Pat. No. 5,242,770 noted above. As
detailed in the '770 patent, it is has been known that adding
assist features around sparse (e.g. isolated) features is necessary
for aggressive printing in photolithography in order to achieve a
manufacturable process. However, the placements of such assist
features are very critical for achieving the optimal and desired
effect. More specifically, similar to adjacent features, it is
possible for incorrect placement of scattering bars around the main
feature to degrade the process latitude of the main feature. For
example, if the scattering bar is placed in a forbidden pitch
region. Accordingly, the present invention can also be utilized for
the placement of scattering bars so as to assure that the
scattering bars are not positioned in a forbidden pitch region for
a given illumination angle.
The implementation of scattering bar technology involves the
determination of scattering bar size and placement. Although the
largest scattering bar size should be used within the resist
contrast capability, the practical design has to take other factors
into account, such as the dimension errors of scattering bars
resulting from the mask-making process. Current scattering bar size
is typically around 60 80 nm. The scattering bar placement is
mainly based on the placement rules that are developed from
experiments, using a specially designed reticle such as MaskTools's
LINESWEEPER.TM. reticle. The principle of scattering bar placement
is similar to that of forbidden pitch phenomena described
above.
More specifically, the first step entails identifying the extreme
interaction locations between the scattering bars and the main
features. The process for identifying the extreme interaction
locations is substantially the same as the process described above
for identifying the extreme interaction pitch regions. However,
instead of identifying small log-slope regions as is necessary for
identification of forbidden pitch regions, regions that show large
log-slope for the main feature are identified. FIG. 13 shows the
extreme interaction edge-to-edge placement positions of the
scattering bars around an isolated main line. As shown, these
extreme edge-to-edge positions are around 235 nm, 375 nm, 510 nm,
655 nm, etc.
Once the extreme interaction locations are identified, the next
step is to select the ones that have a similar illumination map to
the illumination conditions already selected for the process. It is
noted that it is not always true that the closer the scattering bar
is placed around the main isolated feature the better, since each
placement position has its own favorable illumination region. FIGS.
14a d are illumination maps generated for scattering bars having
varying separation from the main feature. Referring to FIGS. 14a
14d, strong scattering bar effects are expected when scattering
bars are placed around 235 nm or 510 nm. However, improper
placement of scattering bars around 375 nm or 650 nm will degrade
the image contrast of the main isolated feature under quadrupole
illumination. If space is provided around the sparse features, a
second pair of scattering bars can be added. When annular
illumination is utilized, constructive and destructive structural
interactions from scattering bars will be averaged out to some
degree, and the benefit from the assist features is therefore
greatly reduced. Thus, it is clear from the foregoing that
placement of scattering bars is strongly dependent on the
illumination chosen. It is also noted that when multiple scattering
bars are required, their illumination maps should belong to the
same class (i.e., the illumination maps should be similar).
To summarize, because both the forbidden pitch phenomena and the
scattering bar technology are a direct consequence of optical
interactions between neighboring features, they can be treated and
understood under a unified framework. The foregoing makes clear
that the field phase of the neighboring feature relative to that of
the main feature depends on the illumination and the separation
distance. For a given illumination angle, there are pitch ranges
within which the field phase produced by the neighboring feature is
180.degree. out of phase relative to the field phase of the main
feature, resulting in destructive interference. Such destructive
interference reduces the image contrast of the main feature, and
therefore causes a loss of exposure latitude. The forbidden pitch
regions, or more precisely the extreme structural interaction pitch
regions, can easily be mapped out and determined as detailed above.
For each extreme structural interaction pitch, a corresponding
illumination map can be obtained, which shows its favorable
illumination regions and its unfavorable illumination regions. The
illumination maps are then utilized as a reference for illumination
design. When the neighboring feature size is changed to the
scattering bar size, similar constructive and destructive
interference regions can be located and their corresponding
illumination maps can also be obtained. Based on these illumination
maps, the optimal scattering bar placement can be determined for a
given illumination condition. Thus, in general, scattering bars
should be placed at the pitch regions where fields from scattering
bars are in phase with the field from the main feature at the main
feature Gaussian image point under a given illumination condition.
When the illumination scheme utilized is changed, the scattering
bar placements should be adjusted accordingly. When multiple
scattering bars are required, their illumination maps should have
similarity to achieve maximum benefit.
As described above, the method of the present invention provides
significant advantages over the prior art. Most importantly, the
present invention provides for identifying and eliminating
forbidden pitch regions, which degrade the overall printing
performance, thereby allowing for an improvement of the CDs and
process latitude obtainable utilizing currently known
photolithography tools and techniques.
FIG. 15 schematically depicts a lithographic projection apparatus
suitable for use with a mask designed with the aid of the current
invention. The apparatus comprises:
a radiation system Ex, IL, for supplying a projection beam PB of
radiation. In this particular case, the radiation system also
comprises a radiation source LA;
a first object table (mask table) MT provided with a mask holder
for holding a mask MA (e.g. a reticle), and connected to first
positioning means for accurately positioning the mask with respect
to item PL;
a second object table (substrate table) WT provided with a
substrate holder for holding a substrate W (e.g. a resist-coated
silicon wafer), and connected to second positioning means for
accurately positioning the substrate with respect to item PL;
a projection system ("lens") PL (e.g. a refractive, catoptric or
catadioptric optical system) for imaging an irradiated portion of
the mask MA onto a target portion C (e.g. comprising one or more
dies) of the substrate W.
As depicted herein, the apparatus is of a transmissive type (i.e.
has a transmissive mask). However, in general, it may also be of a
reflective type, for example (with a reflective mask).
Alternatively, the apparatus may employ another kind of patterning
means as an alternative to the use of a mask; examples include a
programmable mirror array or LCD matrix.
The source LA (e.g. a mercury lamp or excimer laser) produces a
beam of radiation. This beam is fed into an illumination system
(illuminator) IL, either directly or after having traversed
conditioning means, such as a beam expander Ex, for example. The
illuminator IL may comprise adjusting means AM for setting the
outer and/or inner radial extent (commonly referred to as
.sigma.-outer and .sigma.-inner, respectively) of the intensity
distribution in the beam. In addition, it will generally comprise
various other components, such as an integrator IN and a condenser
CO. In this way, the beam PB impinging on the mask MA has a desired
uniformity and intensity distribution in its cross-section.
It should be noted with regard to FIG. 20 that the source LA may be
within the housing of the lithographic projection apparatus (as is
often the case when the source LA is a mercury lamp, for example),
but that it may also be remote from the lithographic projection
apparatus, the radiation beam that it produces being led into the
apparatus (e.g. with the aid of suitable directing mirrors); this
latter scenario is often the case when the source LA is an excimer
laser (e.g. based on KrF, ArF or F.sub.2 lasing). The current
invention encompasses both of these scenarios.
The beam PB subsequently intercepts the mask MA, which is held on a
mask table MT. Having traversed the mask MA, the beam PB passes
through the lens PL, which focuses the beam PB onto a target
portion C of the substrate W. With the aid of the second
positioning means (and interferometric measuring means IF), the
substrate table WT can be moved accurately, e.g. so as to position
different target portions C in the path of the beam PB. Similarly,
the first positioning means can be used to accurately position the
mask MA with respect to the path of the beam PB, e.g. after
mechanical retrieval of the mask MA from a mask library, or during
a scan. In general, movement of the object tables MT, WT will be
realized with the aid of a long-stroke module (coarse positioning)
and a short-stroke module (fine positioning), which are not
explicitly depicted in FIG. 20. However, in the case of a wafer
stepper (as opposed to a step-and-scan tool) the mask table MT may
just be connected to a short stroke actuator, or may be fixed.
The depicted tool can be used in two different modes:
In step mode, the mask table MT is kept essentially stationary, and
an entire mask image is projected in one go (i.e. a single "flash")
onto a target portion C. The substrate table WT is then shifted in
the x and/or y directions so that a different target portion C can
be irradiated by the beam PB;
In scan mode, essentially the same scenario applies, except that a
given target portion C is not exposed in a single "flash". Instead,
the mask table MT is movable in a given direction (the so-called
"scan direction", e.g. the y direction) with a speed v, so that the
projection beam PB is caused to scan over a mask image;
concurrently, the substrate table WT is simultaneously moved in the
same or opposite direction at a speed V=Mv, in which M is the
magnification of the lens PL (typically, M=1/4 or 1/5). In this
manner, a relatively large target portion C can be exposed, without
having to compromise on resolution.
Although certain specific embodiments of the present invention have
been disclosed, it is noted that the present invention may be
embodied in other forms without departing from the spirit or
essential characteristics thereof. The present embodiments are
therefor to be considered in all respects as illustrative and not
restrictive, the scope of the invention being indicated by the
appended claims, and all changes that come within the meaning and
range of equivalency of the claims are therefore intended to be
embraced therein.
* * * * *